[RFC,V5,1/6] mm: Introduce a general RCU get_user_pages_fast.

diff --git a/mm/Kconfig b/mm/Kconfig
index ebe5880..8848a16 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -134,6 +134,9 @@  config HAVE_MEMBLOCK
 config HAVE_MEMBLOCK_NODE_MAP
 	boolean
 
+config HAVE_RCU_GUP
+	boolean
+
 config ARCH_DISCARD_MEMBLOCK
 	boolean
 
diff --git a/mm/Makefile b/mm/Makefile
index b484452..83e6ac2 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -29,6 +29,7 @@  else
 endif
 
 obj-$(CONFIG_HAVE_MEMBLOCK) += memblock.o
+obj-$(CONFIG_HAVE_RCU_GUP) += gup.o
 
 obj-$(CONFIG_BOUNCE)	+= bounce.o
 obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o
diff --git a/mm/gup.c b/mm/gup.c
new file mode 100644
index 0000000..b35296f
--- /dev/null
+++ b/mm/gup.c
@@ -0,0 +1,297 @@ 
+/*
+ * mm/gup.c
+ *
+ * Copyright (C) 2014 Linaro Ltd.
+ *
+ * Based on arch/powerpc/mm/gup.c which is:
+ * Copyright (C) 2008 Nick Piggin
+ * Copyright (C) 2008 Novell Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ */
+
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/rwsem.h>
+#include <linux/hugetlb.h>
+#include <asm/pgtable.h>
+
+#ifdef __HAVE_ARCH_PTE_SPECIAL
+static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
+			 int write, struct page **pages, int *nr)
+{
+	pte_t *ptep, *ptem;
+	int ret = 0;
+
+	ptem = ptep = pte_offset_map(&pmd, addr);
+	do {
+		pte_t pte = ACCESS_ONCE(*ptep);
+		struct page *page;
+
+		if (!pte_present(pte) || pte_special(pte)
+			|| (write && !pte_write(pte)))
+			goto pte_unmap;
+
+		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+		page = pte_page(pte);
+
+		if (!page_cache_get_speculative(page))
+			goto pte_unmap;
+
+		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
+			put_page(page);
+			goto pte_unmap;
+		}
+
+		pages[*nr] = page;
+		(*nr)++;
+
+	} while (ptep++, addr += PAGE_SIZE, addr != end);
+
+	ret = 1;
+
+pte_unmap:
+	pte_unmap(ptem);
+	return ret;
+}
+#else
+
+/*
+ * If we can't determine whether or not a pte is special, then fail immediately
+ * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
+ * to be special.
+ */
+static inline int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
+			 int write, struct page **pages, int *nr)
+{
+	return 0;
+}
+#endif /* __HAVE_ARCH_PTE_SPECIAL */
+
+static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
+		unsigned long end, int write, struct page **pages, int *nr)
+{
+	struct page *head, *page, *tail;
+	int refs;
+
+	if (!pmd_present(orig) || (write && !pmd_write(orig)))
+		return 0;
+
+	refs = 0;
+	head = pmd_page(orig);
+	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
+	tail = page;
+	do {
+		VM_BUG_ON(compound_head(page) != head);
+		pages[*nr] = page;
+		(*nr)++;
+		page++;
+		refs++;
+	} while (addr += PAGE_SIZE, addr != end);
+
+	if (!page_cache_add_speculative(head, refs)) {
+		*nr -= refs;
+		return 0;
+	}
+
+	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
+		*nr -= refs;
+		while (refs--)
+			put_page(head);
+		return 0;
+	}
+
+	/*
+	 * Any tail pages need their mapcount reference taken before we
+	 * return. (This allows the THP code to bump their ref count when
+	 * they are split into base pages).
+	 */
+	while (refs--) {
+		if (PageTail(tail))
+			get_huge_page_tail(tail);
+		tail++;
+	}
+
+	return 1;
+}
+
+static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
+		unsigned long end, int write, struct page **pages, int *nr)
+{
+	struct page *head, *page, *tail;
+	pmd_t origpmd = __pmd(pud_val(orig));
+	int refs;
+
+	if (!pmd_present(origpmd) || (write && !pmd_write(origpmd)))
+		return 0;
+
+	refs = 0;
+	head = pmd_page(origpmd);
+	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
+	tail = page;
+	do {
+		VM_BUG_ON(compound_head(page) != head);
+		pages[*nr] = page;
+		(*nr)++;
+		page++;
+		refs++;
+	} while (addr += PAGE_SIZE, addr != end);
+
+	if (!page_cache_add_speculative(head, refs)) {
+		*nr -= refs;
+		return 0;
+	}
+
+	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
+		*nr -= refs;
+		while (refs--)
+			put_page(head);
+		return 0;
+	}
+
+	while (refs--) {
+		if (PageTail(tail))
+			get_huge_page_tail(tail);
+		tail++;
+	}
+
+	return 1;
+}
+
+static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
+		int write, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pmd_t *pmdp;
+
+	pmdp = pmd_offset(&pud, addr);
+	do {
+		pmd_t pmd = ACCESS_ONCE(*pmdp);
+		next = pmd_addr_end(addr, end);
+		if (pmd_none(pmd) || pmd_trans_splitting(pmd))
+			return 0;
+
+		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd))) {
+			if (!gup_huge_pmd(pmd, pmdp, addr, next, write,
+				pages, nr))
+				return 0;
+		} else {
+			if (!gup_pte_range(pmd, addr, next, write, pages, nr))
+				return 0;
+		}
+	} while (pmdp++, addr = next, addr != end);
+
+	return 1;
+}
+
+static int gup_pud_range(pgd_t *pgdp, unsigned long addr, unsigned long end,
+		int write, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pud_t *pudp;
+
+	pudp = pud_offset(pgdp, addr);
+	do {
+		pud_t pud = ACCESS_ONCE(*pudp);
+		next = pud_addr_end(addr, end);
+		if (pud_none(pud))
+			return 0;
+		if (pud_huge(pud)) {
+			if (!gup_huge_pud(pud, pudp, addr, next, write,
+					pages, nr))
+				return 0;
+		} else if (!gup_pmd_range(pud, addr, next, write, pages, nr))
+			return 0;
+	} while (pudp++, addr = next, addr != end);
+
+	return 1;
+}
+
+/*
+ * Like get_user_pages_fast() except its IRQ-safe in that it won't fall
+ * back to the regular GUP.
+ */
+int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+			  struct page **pages)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long addr, len, end;
+	unsigned long next, flags;
+	pgd_t *pgdp;
+	int nr = 0;
+
+	start &= PAGE_MASK;
+	addr = start;
+	len = (unsigned long) nr_pages << PAGE_SHIFT;
+	end = start + len;
+
+	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
+					start, len)))
+		return 0;
+
+	/*
+	 * Disable interrupts, we use the nested form as we can already
+	 * have interrupts disabled by get_futex_key.
+	 *
+	 * With interrupts disabled, we block page table pages from being
+	 * freed from under us. See mmu_gather_tlb in asm-generic/tlb.h
+	 * for more details.
+	 *
+	 * We do not adopt an rcu_read_lock(.) here as we also want to
+	 * block IPIs that come from THPs splitting.
+	 */
+
+	local_irq_save(flags);
+	pgdp = pgd_offset(mm, addr);
+	do {
+		next = pgd_addr_end(addr, end);
+		if (pgd_none(*pgdp))
+			break;
+		else if (!gup_pud_range(pgdp, addr, next, write, pages, &nr))
+			break;
+	} while (pgdp++, addr = next, addr != end);
+	local_irq_restore(flags);
+
+	return nr;
+}
+
+int get_user_pages_fast(unsigned long start, int nr_pages, int write,
+			struct page **pages)
+{
+	struct mm_struct *mm = current->mm;
+	int nr, ret;
+
+	start &= PAGE_MASK;
+	nr = __get_user_pages_fast(start, nr_pages, write, pages);
+	ret = nr;
+
+	if (nr < nr_pages) {
+		/* Try to get the remaining pages with get_user_pages */
+		start += nr << PAGE_SHIFT;
+		pages += nr;
+
+		down_read(&mm->mmap_sem);
+		ret = get_user_pages(current, mm, start,
+				     nr_pages - nr, write, 0, pages, NULL);
+		up_read(&mm->mmap_sem);
+
+		/* Have to be a bit careful with return values */
+		if (nr > 0) {
+			if (ret < 0)
+				ret = nr;
+			else
+				ret += nr;
+		}
+	}
+
+	return ret;
+}